Electrical sensing apparatus



Feb. 9, 1960 ELAM Re. 24,779

ELECTRICAL SENSING APPARATUS Original Filed Jan. 11, 1955 2 Sheets-Sheet 1 l k g 21c: pan Of 6&4

fi v mpd MMW Mg/J Feb. 9, 1960 E ELECTRICAL SENSING APPARATUS 2 Sheets-Sheet 2 Original Filed Jan. 11, 1955 United States Patent" ELECTRICAL SENSING APPARATUS David L. Elam, Roselle, Ill., assignor to Electro Products Laboratories, Inc., a corporation of Illinois Original No. 2,883,538, dated April 21, 1959, Serial No. 481,146, January 11, 1955. Application for reissue August 3, 1959, Serial No. 831,449

6 Claims. (CI. 331 -64) Matter enclosed in heavy brackets II] appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

This application is concerned with a sensing apparatus and more particularly with an electircal apparatus for sensing the presence of an object of a conductive material.-

One feature of the invention is the provision of a sensing apparatus comprising an electrical oscillator including a tuned circuit having an inductance therein, means for normally maintaining oscillations in the oscillator,.a pickup unit including at least a portion of the inductance, the oscillations of the oscillator being aifected by the presence of a conductive object adjacent the pickup and in the field of the inductance and means responsive to the oscillations for deriving an indication of the presence or absence of a conductive object in the field of the inductance. Another feature is that the tuned circuit of the oscillator has a very low Q. A further feature is that provision is made for varying the sensitivity of the oscillator.

Another feature is that the tunedcircuit of the oscillator comprises an inductance and distributed capacity with no lumped capacity. Yet a further feature is that the pickup unit, including the inductance is connected at the end of a multi-conductor cable and that the distributed capacity of the cable provides a major portion of the capacity of the tuned circuit.

Still a further feature is that the pickup unit includes a mountable housing of conductive material and having a bore, a sleeve of insulating material carried in the bore and extending outwardly therefrom and a coil form inside the sleeve, the inductance being wound on the coil form and outwardly of the housing.

Further features and advantages will readily be apparent from the following specification and from the drawings, in which:

Figure l is a diagrammatic view showing the sensing apparatus in use as a counter;

Figure 2 is a diagrammatic view showing the sensing apparatus in use as a distance measuring instrument;

Figure 3 is a schematic diagram of the sensing apparatus;

' Figure 4 is a schematic diagram of a modified output circuit for the apparatus;

Figure 5 is a fragmentary elevational view of the pickup unit;

Figure 6 is an enlarged view similar to Figure 5 with a portion of the pickup unit broken away;

Figure 7 is an enlarged sectional view taken along line 7-7 of Figure 5; and

Figure 8 is a transverse sectional view taken along line 8-8 of Figure 7.

Many types of electrical sensing apparatus are known and in use today. They are used in conjunction with counters,'measuring devices, sorting equipment and metal detectors, to name only a few. Different types of sensing apparatus operate on different principles, some utilizing photo cells, others an induced voltage resulting from the I Re. 24,779 Reissued Feb. 9,1960

ice

variation of the flux linkages of a coil and with others depending on a capacity variation.

. The present invention is concerned with a novel and miproved sensing apparatus which operates on the basis of an eddy current loss and may be used with any material which is conductive.

Referring now to Figures 1 and 2, the electrical sensing apparatus indicated generally as 10is shown as it may be used in two difierent sensing operations. In Figure 1 the apparatus is utilized to count the number of bottles 11 moving along a conveyor 12. The pickup unit 10a, ,which is connected to the apparatus by a cable 10b, is mounted adjacent the tops of the bottles 11. As each bottle passes the pickup unit 10a the sensing apparatus is triggered by the conductive metal caps 11a, actuating counter 13.

In Figure 2, the sensing apparatus is used as a measuring instrument, the pickup unit 10a being mounted in a desired location. At some point as an object 14 of a conductive material is moved toward pickup 10a and the sensing apparatus is triggered giving an indication as by energizing a light 15. The particular distance at which an indication is given may be varied by an adjustable control 16. The sensing apparatus for this use is initially adjusted by placing an object 14 the desired predetermined distance from the pickup 10a and setting control 16 to give the desired indication. After this, objects 14 may be moved into the desired position with great accuracy by observing indicator 15. This last embodiment of the sensing apparatus may, for example, be usedin operating large machine tools to gauge the depth of a cut or the like or may be incorporated as a limit switch in automatic or semiautomatic tools.

Referring now to the schematic diagram, Figure 3, the circuit of the sensing apparatus will be described in detail. The particular values and tube types given are intended to be exemplary and are not critical unless specifically indicated otherwise.

The sensing unit may be energized by connecting leads 20 to a suitable source of power as 110 volts, AC. The primary 21a of power transformer 21 is excited on closing of a switch 22. A half wave rectifier Power supply including a selenium rectifier element 23, filter capacitors 24, 25 and 26, 20 uf. (microfarads), 30 of. and 10 i, respectively, and resistor 27, 5.6 KS2, together with bleeder resistor 28, 47 Kn, and potentiomer 29, 10 Kit, is connected to secondary winding 21b of the power transformer. Secondary winding 21c provides a suitable heater potential for the tubes of the unit.

The basic element of the sensing apparatus is a Hartleyconnected triode oscillator 30 which may utilize one-half of a 6SN7. The tuned circuit for oscillator 30 includes an inductance 31, which is in the pickup unit 10a and the distributed capacitance of the system, the major portion of which is in the pickup cable 10b. The tuned circuit is connected between grid 32 [and cathode 33] of oscillator 30 and ground through the shield of cable 10b. A tap 31a on the oscillator coil is connected with cathode 33 of the oscillator. [returned to ground through the shield of cable 10b' Cathode resistor 34, 2,000 n is connected in the circuit of cathode 33 providing an operating bias for the oscillator and loading the tuned circuit. Anode 35 of oscillator 30 is connected to movable arm 29a of potentiometer 29 providing a variable operating potential for the oscillator.

The output of oscillator 30 is coupled directly from the cathode 33 thereof to the grid 37 of an amplifier 36, which may be the other half of the 6SN7. Cathode 38 of amplifier 36 is returned to ground through b as resistor 39, 1,000 n and capacitor 40, .05 t. Anode 41 is connected through load resistor 42, 56 Kn to a fixed B+ potential provided by the power supply. The oscillations 36 are couple'dthrough capaictor 43, .0005 t, to the grid 44 of a second amplifier 45 which may also be onehalf: of a 6SN7. Grid 44 of amplifier 45 is returned to ground through resistor 46, IOO'KQ while cathode 47 is connected to ground through cathode resistor 48', 1,000 o, and a capacitor 49, .05 f. The anode 50 of amplifier 45 is also connected to the B+ power supply through a 56 K9 load resistor,'51. The output of amplifier 50 is coupled through capacitor 52, .0005 ,uf, to a detector circuit including a 1N54 germanium diode 53, resistor 54, 100 K9, and capacitor 55, .0015 ,uf. The grid 60 of an output amplifier 61,,which may also be one-half of a 6SN7, is connected to the juncture of a germanium diode 53 of capacitor 55 while cathode 62 is returned to ground through load resistor 63, 10 KS2. Anode 64 is connected directly to the B+ power supply.

Under normal conditions, that is without a conductive object to be sensed near the pickup 10a, oscillator 30 oscillates continuously andthe output thereof is amplified by amplifiers 36 and 45. These amplified oscillations are rectified by the germanium diode 53 applying a sufficient' negative bias potential to the grid. 60 of output amplifier 64 tocutofl? this stage. Whena mass of conductive material is introduced into the field of oscillator coil31 the eddy current. losses occurring therein serve to damp or squelchcthe oscillations in. oscillator 30, removing the bias from output amplifier 61 andpermitting a substantial current to flow therethrough. The resultingvoltage pulse obtained acrosscathode resistor 63 may be coupled, as through a cable 65 to a desired indicating or utilizing de vice. As soon as the conductive object is removed from the field of coil 31, oscillations resume in oscillator 30 and output amplifier 61 is again cutofi.

Referring now more particularly to Figures -8, the pickup unit and oscillator coil assembly will be described in detail. The pickup unit includes an. elongated, exteriorlythreaded, housing 70 having a longitudinal opening or bore 70a therein. A nut 71 mating with the threadedhousing permits mountingof the pickup, as by inserting it through an opening inva plate and then tightening the, nut. Received within bore 70a of housing 70 is a'sleeve' 72 of insulating material, as asynthetic plastic, and which extends outwardly beyond the end of housing 70 about one-half inch- Inside sleeve 72 is .acoil form 73 having an end portion 73a which closes the open. end Ofwthfi sleeve 72. Inductance 31 is wound in av narrow. groove: 73b provided immediately inside end portion. 73a and well beyond the end of metal housing. 70.. In the circuit describedabove, coil 31 comprises.110 turns of No. 40. wire, random wound, with a center tap for connection to the pickup cable shield. The three connections to coil :31. are-brought out of groove 73b through longitudinal slots74, of which there are three, into a wide groove 75. In groove 75 the connections to coil31 are soldered to heavy lead wires 76 and which inturn .are connectedtotconductors of cable b. Both wires 76 andthe ing from the. spirit and scope of the invention as disclosed presence of conductive material in the field of coil 31. The Q 'ofthe tuned circuit described is less than one.

Figure 4 shows a modified output circuit in which a relay 91 shunted by capacitor 92 is connected in the plate circuit of output amplifier. 61,.replacing the cathode load resistor shown in Figure 3. The operation of the detector circuit including germanium diode 53, resistor 54 and capacitor is the same as that previously described. So long as the oscillator is in normal operation, amplifier 61 is cut off'and'no plate current flows. The movable contact arm 91atof v relay 91 is inthe position shownin thev drawings completing a circuit between output. terminals 93 and 94. When a.conductive material is introduced into the field of coil 31,. squelchingoscillator 30, output amplifier 61 conducts heavily, energizing relay 91 whereupon contact 91a moves: downwardly completing ;thew circuit between terminals 93and 95'. Of course; any desired circuit' as a counter, control device or the like may be connected to the output terminals 93, 94-and* 95 ofrelay 91.

The variable anode potential provided for'oscillator30 by virtue of its connection to movable arm 29a of po tentiometer 29 provides a means for varying thesensitivity of the oscillator by. varying an operating condition" of'the tube. The anode voltage for the oscillator maybe adjusted'as discussedxabove in'connection'with sensitivity control 16 shown in Figures: '1 and 2, providing'the sensitivity necessary fora'particular use of'the sensing apparatus. The sensitivity control permits triggeringthe' oscillator at different distances between thepickup and conductive materials or metallic bodies. It also can be used to take care of different size masses of conducting material from a. triggering standpoint.

The circuit of Figure 3 with the output taken across cathode resistor 63 of amplifier 61 provides a linear, un-' distorted output which isfsubstantially a. square wave in shape at frequencies'up to about 1500 c.p.s. The response of the indicator circuit of Figure 4 is of course limited by the mechanical capabilities of relay 91. The use of. an'oscillatoroperating frequency in the radio frequency range improvesthe frequency response of the system by permitting the oscillator a faster quench 'and'recovery time while allowing the use of small circuit components.

While I have shown and described certain embodiments of my invention, it. is to be understood that it is capable of many modifications. Changes therefore, in the construction and arrangement may be made without departin the appended claims.

I claim:

1. Sensing apparatusof the character described, comprising: fixed. radio frequency electrical oscillator includ- I ing a low Q tuned circuit having an inductance therein;

le'adsof coil 31 are tightly wrappedwin grooves 75 as by t As previously mentioned, oscillator coil 31 is tuned entirely by thedistributed capacitance of the circuit,.and primarily that of pickup cable. 10b. The cable used in theyexemplary circuit istaatwo conductor shielded cable ten feet long: The cable'has'a; distributed capacitance of 25 ,u Lf. per foot, and a total capacity of 250 unit The operating frequency of the oscillator 30 is. preferably of the order of severalhundred kilocycles per secondiand in the circuit which hasbeen. described is-between-700 and 800 kilocycles. The Q of the tuned circuit is intention-v ally made extremely'low, preferably less than 10,.in order to: provide a sensitive oscillator circuit which may be damped. or; squelched completely .by the losses due 5 to .the

meansfor normally maintaining oscillationsjn. said oscillator; a pickup unit including substantially all. saidinductance, the oscillations of said. oscillator being damped by the presence. ofa conductive .object adjacent said'pickup and,in. the field of said inductance; and: means, connected to said oscillator and responsive to said oscillations for deriving anindication of. the presence or. absence of 'a conductive object in the fie'ldofsaid inductance.

2.. Sensing, apparatus of the character described in claim 1, wherein saidtuned circuit has a Q of less than 10.

3.. Sensingapparatus ofthe character described,'comprising: an electrical. oscillator including a tuned circuit made .up.ofaninductanceand only distributed capacity; means for normally maintaining oscillations in said oscil: lator;. a pickup unit, including substantially all said-inductance, the oscillations of'said oscillator being quenched by the presence of a conductive object adjacent said pickup. and in the field of'said inductance; and means responsive to said oscillations for deriving an indication claim 3, wherein said pickup unit is connected to the oscillator by an elongated multi-conductor cable and said distributed capacity is made up substantially only of the distributed capacity of said cable.

5. Sensing apparatus of the character described, comprising: an oscillator including a tube having an anode, a control grid and a cathode; a tuned circuit, including an inductance, connected between said control grid and cathode; means connected to said anode providing operating potentials for causing said tube to oscillate, the connection of said tuned circuit to said cathode including a resistive element providing degenerative feedback, the oscillations of said tube being quenched by the presence of a conductive object in the field of said inductance; and means responsive to said amplified oscillations for deriving an indication of the presence or absence of a conductive object in the field of said inductance.

6. Sensing apparatus of the character described, com prising: an electrical oscillator including a tuned circuit having an inductance; means for normally maintaining oscillations in said oscillator; a pickup unit including a mountable housing of conductive material having a bore therein, a sleeve of insulating material carried in said bore and extending outwardly therefrom, and a coil form inside said sleeve, and having a portion closing the end 2 thereof, said inductance being wound on said form, outwardly of said housing and adjacent said portion, the oscillations of said oscillator being affected by the presence of a conductive object adjacent the pickup and in the field of the inductance; and means responsive to said oscillations for deriving an indication of the presence or absence of a conductive object in the field of said inductance.

References Cited in the file of this patent or the original patent UNITED STATES PATENTS 2,267,884 Zuschlag Dec. 30, 1941 2,442,805 Gilson June 8, 1948 2,443,125 Weathers June 8, 1948 2,537,065 Lester et a1. Jan. 9, 1951 2,576,173 Cornelius Nov. 27, 1951 2,580,670 Gilbert Jan. 1, 1952 2,629,004 Greenough Feb. 17, 1953 2,660,704 Harmon et a1 Nov. 24, 1953 2,671,173 Gamersfelder Mar. 2, 1954 2,729,785 Keevil Jan. 3, 1956 2,772,394 Bradley Nov. 27, 1956 2,774,060 Thompson Dec. 11, 1956 2,778,574- Moore et al Ian. 22, 1957 OTHER REFERENCES Journal of Scientific Instruments, vol. 21, pp. 108, 109, June 1944, An Electronic Indicator for Liquid Separation, J. W. Broadhurst.

Handbook of Industrial Electronic Circuits, McGraw- Hill, pages 44,128-133. 

